28 ZOOLOGY. 



trunk which curves round to the left side, forming an arch ; 

 that from the left ventricle is the anterior of the two, and 

 is called the aortic or systemic arch \ the other, from the 

 right ventricle, is the pulmonary arch. Where these arches 

 curve back they come close to one another again, and here 

 there occurs a connexion between them, called the ductus 

 arteriosus. This connexion is a solid cord in the adult 

 rabbit, and perfectly useless; in a newly-born rabbit it 

 is much larger, and before birth it is a true artery, through 

 which the blood flows from the pulmonary arch into the 

 systemic. From the pulmonary arch of the adult come off 

 two arteries only the right and left pulmonary arteries, 

 which take blood to the right and left lungs. 



11. The Double Circulation. The student will notice 

 that the blood leaving the right side of the heart goes 

 to the lungs only ; while that leaving the left side goes 

 to all other parts of the body. On the other hand, all the 

 blood entering the left side comes from the lungs only, while 

 that entering the right side comes from all other parts. 

 Hence, since the right and left sides of the heart are 

 completely separated internally, there is in the rabbit (and 

 all mammals) a complete double circulation. In order that 

 a blood-corpuscle, leaving the capillaries of a given organ, 

 may get back to the same capillaries again, it must pass 

 through the heart twice, or some even number of times 

 first through the right side and then through the left, and 

 so on alternately. And each time, after passing through 

 the right side and before it can pass through the left, 

 it must pass through the capillaries of the lungs, and these 

 capillaries only. 



In the embryo the lungs are not in use, and there is no 

 need for a double circulation. Thanks to the ductus 

 arteriosus, most of the blood from the right side passes 

 straight to the dorsal aorta without having to go through 

 the lungs and again through the heart. Structures like 

 the ductus arteriosus, that are useless in the adult stage, 

 are called vestiges. 



12. The Heart's Action. The beating of the heart is, 



